1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device wherein fixed data is stored beforehand.
2. Description of the Related Art
Among the conventional semiconductor integrated devices for microcomputers, there is a type which stores a fixed program beforehand. The fixed program contains instruction codes which fetches external program data during the initial operation performed immediately after the power supply is turned on. The memory area for storing the fixed program is provided on the same semiconductor substrate as ordinary memory areas (e.g., a RAM, a ROM, an EPROM and an EEPROM), but it is constituted by an exclusive-use nonvolatile memory provided independently of the other memory areas.
Among the conventional semiconductor integrated devices for microcomputers, there is also known a type which permits signatures (such as information on a manufacturer, a device, etc.) to be read out and output to an external device in response to entry of a specific code. Examples of such a device are the EPROM integrated circuit devices disclosed in U.S. Pat. Nos. 4,419,747 and 4,451,903. In this type of semiconductor integrated devices, a memory area in which the fixed data is programmed is located adjacent to an externally-programmable memory area.
In the former type of device, since the memory area for storing the fixed program is provided independently of the other memory areas, a cell area, a decoder circuit area, an input/output circuit area, etc. must be additionally provided for that memory area. Even when the memory area does not have a large storage capacity in comparison with the storage capacities of the other memory areas, the installation area required for the memory area is very wide, resulting in an increase in the manufacturing cost of the entire device.
In the former type of device, the memory area for storing the fixed program is provided independently of the other memory areas, although it can be realized by the same circuitry as the other memory areas. Therefore, the device is redundant in system configuration. In addition, the operation speed of the memory area for storing the fixed program inevitably differs from that of the other memory areas.
In the latter type of device, the externally programmable memory area is located adjacent to the fixed program memory area. Even in this case, performance such as the operating speed greatly varies, depending upon the circuit pattern employed.
The programmable memory area adjacent to the fixed program memory area can be realized by either a volatile type or a nonvolatile type. In practice, however, the programmable memory area is made by a nonvolatile memory, such as an EPROM, and the use of the fixed program memory area is limited to a particular purpose, such as the storage of signatures. Although fixed data can be written by external operation, the data stored beforehand may be damaged or undesirably altered due to an erroneous operation.
As can be understood from the above, in the conventional devices, the fixed program to be executed immediately after the power supply is turned on must be stored in a memory area provided independently of the other memory areas. Thus, a cell area, a decoder circuit area and an input/output circuit area are additionally required only for the fixed program memory area. Even when these memory area does not have a large storage capacity in comparison with the storage capacities of the ordinary memory areas, the installation area required is very wide, resulting in an increase in the manufacturing cost of the device.